Carbon dioxide flooding processes are an important enhanced oil recovery method to recover oil from reservoirs, including in both sandstone and carbonate reservoirs. Traditionally, approximately one third of the original oil in place is recovered by primary and secondary recovery processes. However, this typically leaves two-thirds of the oil trapped in reservoirs as residual oil after water flooding. An additional five to twenty percent of the oil may be recovered by carbon dioxide flooding processes. However, increasing the recovery beyond this has remained difficult because of several challenges. First is the gravity override of the injected carbon dioxide due to density differences between the injected carbon dioxide and resident fluids in the reservoir. The carbon dioxide, being lighter, tends to rise to the top of the reservoir, thereby bypassing some of the remaining oil. This results in poor oil recovery in the lower portion of the reservoir. This problem is especially acute in thick formations. The second challenge is viscous fingering that is caused by the lower viscosity of the injected carbon dioxide. Typical dense carbon dioxide viscosity at reservoir conditions is in the range of 0.05-0.1 cP, which is much lower than the viscosity of resident oil and brine. The resulting unfavorable mobility ratio leads to viscous fingering. This causes early carbon dioxide breakthrough, high carbon dioxide utilization factors, poor sweep efficiency and low overall oil recoveries. The third challenge is reservoir geology and heterogeneities, including high permeability streaks and fractures that can affect the sweep efficiency of a carbon dioxide enhanced oil recovery flooding processes. While traditional water-alternating-gas processes have shown to improve the mobility of carbon dioxide somewhat, traditional water-alternating-gas processes have not completely overcome these challenges.
Increasing the density and viscosity of carbon dioxide can alleviate many of these challenges and lead to substantially higher recovery than conventional carbon dioxide enhanced oil recovery processes. Carbon dioxide density can be increased by blending in heavier compatible materials. However, historically, limited success has been achieved using this approach.
Additionally, known methods use surfactants to foam or to create water in carbon dioxide reverse micelles. While creating a foam addresses the challenge of viscosity, it leaves the challenge of density unresolved. Although research results have demonstrated that surfactant-induced carbon dioxide foams are an effective method for mobility control in carbon dioxide foam flooding, the foam's long-term stability during a field application is difficult to maintain.
Moreover, even if a carbon dioxide thickener, whether a polymer or small molecule, is identified for use in enhanced oil recovery processes, operational constraints may be encountered by operators who would try to implement the technology in a pilot-test. Nearly all potential carbon dioxide thickeners are a solid at ambient temperature and a means of introducing a powder into the carbon dioxide stream must be employed, possibly by first dissolving the thickener in an organic solvent in order to form a concentrated, viscous, pumpable solution.
It is known the solubility of carbon dioxide in oil is greater than that of the solubility in water. Additionally, the diffusion rate of carbon dioxide from degassed oil to live oil in two similar phases is much higher than the diffusion rate of carbon dioxide from water to live oil.
The features and advantages of the present invention will be readily apparent to those skilled in the art upon a reading of the description of the preferred embodiments that follows.